Mark Kleiman considers Washington's and Colorado's new laws:

The Colorado measure is much more radical than the Washington measure. Both intend to establish regulated markets on the alcohol model, something the federal government could probably shut down with relatively modest effort if it decided to. But Colorado also legalizes every resident to grow his or her own marijuana, and to give away as much as an ounce at a time to others. "Give away" is an interesting category; an informal market is likely to emerge, on a "Pay me $20 for this zip-lock baggie and I’ll give you the green stuff inside" basis.

While the feds could easily identify a limited number of state-licensed growers and retail outlets and shut them down with injunctions or with threats of arrest and property confiscation, identifying and cracking down on an unknown number of unlicensed home-growers would be next to impossible. So Colorado’s voters may well have let the genie out of the bottle in a way that no federal action can now reverse.

Earlier commentary on the subject here and here.